We are using the assembly of the head of bacteriophage T4 as a model for understanding processes of synthesis and regulation of formation of intracellular structures. The regulation of limited proteolysis carried out by T4 during capsid maturation is under investigation. We have purified the bacteriophage enzyme responsible for the proteolysis (T4PPase) and are now studying the properties of the enzyme. We have shown the specificity of cleavage of this phage protein to be glu-X, where X may be ala, glu or asx. Studies now in progress show that cleavage sites are also determined by the folding of the substrate proteins, and we are investigating this in detail, since so far only phage proteins have been found to be substrates for T4PPase. We have found that T4PPase is synthesized as a zymogen, the product of gene 21, which is activated by proteolysis in the mature prehead. The control of this activation is also under study. BIBLIOGRAPHIC REFERENCES: Steven, A.C., U. Aebi, and M.K. Showe 1976. Folding and Capsomere Morphology of the P23 Surface Shell of Bacteriophage T4 Polyheads from mutants in five different head genes. J. Mol. Biol 102, 373-407. Steven, A.C., E. Couture, U. Aebi, and M.K. Showe 1976. Structure of T4 Polyheads. II. A pathway of Polyhead Transformations as a model for T4 capsid maturation. J. Mol. Biol 106, 187-221.